A long chain made of many repeating units

A small molecule with a double bond

Polymers | HSC Chemistry Module 7

In short: A polymer is a long chain made of many small repeating molecules called monomers. Addition polymers like polyethene form from alkenes with nothing else made, while condensation polymers like nylon and polyesters give off water as each link forms.

Monomers and polymers

A monomer is a small molecule that can join to others.
A polymer is a very long molecule made of many monomers joined by covalent bonds.
Polymerisation is the reaction that joins them.

There are two kinds: addition and condensation.

Addition polymers

Monomers with a C=C double bond open the double bond and join end to end. Nothing else is produced.

$n C H_{2} = C H_{2} \to - (C H_{2} - C H_{2})_{n} -$

ethene → polyethylene

Polymer	Monomer	Properties	Uses
Polyethylene (PE)	ethene $C H_{2} = C H_{2}$	Flexible, cheap, waterproof	Bags, bottles, cling wrap
Polyvinyl chloride (PVC)	chloroethene $C H_{2} = CHCl$	Strong, rigid, waterproof, electrical insulator	Pipes, window frames, cable coating
Polystyrene (PS)	phenylethene (styrene)	Rigid; can be expanded into a light foam	Foam cups, packaging, insulation
PTFE (Teflon)	tetrafluoroethene $C F_{2} = C F_{2}$	Non-stick, very unreactive, high melting point	Non-stick pans, plumbing tape

Drawing the polymer: draw the monomer's carbon chain with the C=C as a single bond, put brackets around one repeat unit, and add "n". For three repeat units of PVC, draw six carbons in a row with a Cl on every second carbon.

LDPE and HDPE

Polyethylene comes in two forms, depending on how it is made:

	LDPE (low density)	HDPE (high density)
Chains	Lots of branches	Few branches, straight chains
Packing	Chains cannot pack closely	Chains pack tightly, in neat crystalline regions
Dispersion forces	Weaker	Stronger
Properties	Soft, flexible, lower melting point	Harder, stronger, more rigid, higher melting point
Uses	Plastic bags, cling wrap	Milk bottles, buckets, pipes

Condensation polymers

Monomers with two functional groups (one at each end) join, and a small molecule (usually water) is released at every link.

Polyesters form from a diol (two −OH groups) and a dicarboxylic acid (two −COOH groups). Each link is an ester group. Example: PET, used in drink bottles and polyester clothing.

Polyamides form from a diamine (two −NH₂ groups) and a dicarboxylic acid. Each link is an amide group, −CONH−. Examples:

Nylon 6,6: strong, elastic fibres for clothing, ropes and carpets.
Kevlar: ring-shaped monomers make rigid, flat chains that line up and hydrogen bond strongly to each other. It is five times stronger than steel by weight and is used in bulletproof vests.

Natural condensation polymers include proteins (polyamides of amino acids), cellulose and starch (from glucose).

A single monomer with both groups can also polymerise by itself. For example, lactic acid (with −OH and −COOH) forms PLA, a biodegradable polyester.

Structure and properties

Feature	Effect
Longer chains	More dispersion forces, so stronger and higher melting point
Branching	Looser packing, so softer and lower melting point
Polar groups (like C−Cl in PVC)	Dipole-dipole forces, so more rigid
Hydrogen bonding between chains (nylon, Kevlar)	Very strong fibres
Bulky side groups (polystyrene)	Stiff chains

Plastics and the environment

Most synthetic polymers are made from crude oil (non-renewable).
They are not biodegradable, so they last for hundreds of years in landfill and oceans, and break into microplastics.
Burning PVC releases toxic HCl gas.
Recycling and biopolymers (such as PLA, or PHB made by bacteria from sugar) help. Biopolymers come from renewable sources and break down.

Polymers

Basics

Monomer, polymer
Repeat unit, n

Addition

C=C monomers
No by-product
PE, PVC, PS, PTFE

LDPE vs HDPE

Branched, soft
Straight, strong

Condensation

Two functional groups
Water released
Polyester: diol + diacid
Polyamide: diamine + diacid
Nylon, Kevlar, PET

Properties

Chain length
Branching
Hydrogen bonds

Environment

From oil
Not biodegradable
Biopolymers

Year 11Molar mass of a polymer

The molar mass of an addition polymer is the monomer's molar mass times the number of units:

$M M (polymer) = n \times M M (monomer)$

Example. A sample of PVC has a molar mass of 125 000 g/mol. Chloroethene, $C_{2} H_{3} Cl$ , has a molar mass of 62.49 g/mol. So $n = 125000 \div 62.49 = 2000$ monomer units.

From polymer back to monomer. Find the repeating two-carbon unit and put the double bond back. $- C H_{2} - CH (C H_{3}) - C H_{2} - CH (C H_{3}) -$ repeats $C H_{2} CH (C H_{3})$ , so the monomer is propene, $C H_{2} = CHC H_{3}$ .

No double bond left. Polyethylene has only single bonds, so it does not decolourise bromine water, while ethene does.

Same percentage of carbon. An addition polymer is just its monomer units joined, with no atoms lost, so it has exactly the same percentage of carbon by mass as the monomer.

Year 12Condensation calculations and Kevlar

In a condensation polymer, each new link loses a water molecule, so the repeat unit is lighter than the monomers that made it.

Example. 4-aminobutanoic acid, $H_{2} N (C H_{2})_{3} COOH$ (103.12 g/mol), polymerises. Each repeat unit is the monomer minus $H_{2} O$ : $103.12 - 18.02 = 85.10$ g/mol. A chain of 100 units has a molar mass of about $100 \times 85.10 = 8.5 \times 1 0^{3}$ g/mol.

Why Kevlar is so strong. Its benzene rings keep each chain straight and rigid. The chains line up side by side, and the N−H of one chain hydrogen bonds to the C=O of the next. Millions of these hydrogen bonds make a sheet-like structure that resists stretching. Nylon has flexible $C H_{2}$ sections, so it stretches more.

Two monomers. For nylon 6,6, one repeat unit comes from one diamine and one diacid, losing two waters: $116.20 + 146.14 - 2 \times 18.02 = 226.30$ g/mol.

Counting units. Divide by the repeat unit mass, not the monomer mass. PHB's monomer is 104.10 g/mol, so its repeat unit is $104.10 - 18.02 = 86.08$ g/mol, and a chain of $8.6 \times 1 0^{4}$ g/mol has about 1000 units.

Quick summary

Polymers are long chains of repeating monomers.
Addition polymers form from alkenes; nothing else forms. Examples: PE, PVC, PS, PTFE.
LDPE is branched and soft; HDPE has straight chains and is stronger.
Condensation polymers release water: polyesters (diol + diacid) and polyamides (diamine + diacid).
Hydrogen bonds between chains make nylon and Kevlar strong.
Most plastics come from oil and do not biodegrade; biopolymers are an alternative.

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Try 3 questions now

A polymer is:
1. A A long chain made of many repeating units
2. B A small molecule with a double bond
3. C An ionic compound
4. D A mixture of metals
Show answer
A: A long chain made of many repeating units
Poly means many.
Addition polymers form from monomers that contain:
1. A An −OH group
2. B A C=C double bond
3. C Two −COOH groups
4. D An amide group
Show answer
B: A C=C double bond
The double bond opens to link the monomers.
The monomer of polyethylene is:
1. A Ethane
2. B Ethene
3. C Ethanol
4. D Ethyne
Show answer
B: Ethene
$C H_{2} = C H_{2}$ .